Density sensors are utilized to convert a buoyant force into an electrical signal through the use of a sealed float coupled to some type of transducer. To gain a high degree of accuracy, density sensors in the past have utilized a firm mechanical coupling to a transducer such as a load cell. This firm mechanical linkage allows the buoyancy force to be directly translated from the float to the load cell. Although this method of measuring the buoyant force is very accurate, it has the disadvantage of being very fragile. This is undesirable in applications in which these devices are submitted to environments with buoyant forces that have a high level of acceleration components associated therewith. These acceleration forces are the result of shock load forces that the float is subjected to in a working environment. These shock load forces subject the associated transducers to both very high and very low frequency acceleration force components that are particularly damaging to the transducers.
The transducers normally utilized in density sensors are the load cells which are basically a loaded beam attached to which are strain gauges. The strain gauge is an electro-mechanical device that converts strain into a resistive change. By incorporating this strain sensitive resistance into a balanced bridge circuit, an electrical signal can be generated that is proportional to the strain that the strain gauge is subjected to. To couple strain to the strain gauge from, for example, a loaded beam, it is necessary to utilize some form of adhesive. This adhesive is disposed between the loaded beam and the strain gauge and is subject to strain therein. This adhesive is the point at which strain gauges incur the most failures. Acceleration components contained in the shock load forces are readily transmitted through both the strain gauge and the loaded beam due to the high inertia thereof. However, the adhesive disposed therebetween has a much lower inertia resulting in very poor coupling of strain to the strain gauge. The result is that the adhesive fails and, consequently, the transducer must be replaced.
An example of a harsh environment in which density sensors are utilized is a mud pit in an oil field. These mud pits are typically subjected to very high turbulences which are required to maintain an equal density therein. Normally, a dedicated impeller is continually running to maintain this high turbulence. When density sensors are submerged in this type of environment, the high turbulences therein subject the density sensors to high shock load forces with components of very high frequency accelerations and also some low frequency acceleration components. These high frequency acceleration components are probably some of the most damaging forces that the density sensor can be subjected to, thereby resulting in very short mean time to failure rates for presently available density sensors. The low frequency acceleration components are primarily due to unusual forces, such as dropping the density sensor or allowing a hard object to impact the float on the density sensor. Since most density sensors utilize a fixed mechanical linkage between the float and the transducer, these high and low frequency acceleration forces are directly transmitted to the transducer thereby increasing the failure rates thereof.
In view of the above disadvantages, there is a need for a density sensor that is impervious to the components of high and low frequency accelerations that are present in the buoyant forces present in the harsh operating environments.